1. Field of the Invention
This invention relates to means for preventing cooling gas leakage from the housing of a gas-cooled dynamoelectric machine and more particularly to a fluid supply system which maintains the required sealing fluid flow to a set of seal plates by detecting a variety of malfunctions and initiating corrective action.
2. Description of the Prior Art
In hydrogen-cooled turbine generators the rotor shaft ends must be brought out of a gas-tight enclosure, necessitating the use of some means to prevent escape of the gas along the shaft. Seal plates which are supplied with oil under pressure are used for this purpose. Oil is pumped to a feed groove between the annular seal plates. When the oil pressure in the feed groove exceeds the gas pressure in the turbine generator, some oil will be forced both ways along the shaft through the small clearance space between the seal plates and the shaft thus preventing escape of hydrogen gas from the turbine generator. The seal plates' function is to restrict the flow of oil through the sealing clearance space. These plates can move radially with the shaft, but are restrained from rotating.
The system supplying oil to the seal plates must maintain the oil pressure at some value above the hydrogen gas pressure in the turbine generator. It is also necessary to provide redundant means for supplying oil to the seal plates because loss of oil to them may cause extensive hydrogen leakage from the generator and damage to both the seal plates and shaft due to frictional heating caused by a lack of lubrication between them. An example of a sealing scheme used for hydrogen-cooled dynamoelectric machines is that of C. C. Sterrett, U.S. Pat. No. 2,159,057. Although a backup pump is provided by Sterrett in case of primary pump failure, there are no means to supply sealing oil to the seal plates during the time lag experienced in bringing the backup pump to its rated flow and pressure.
In present seal oil systems a constant pressure backup supply of seal oil is often provided by the turbine lubrication or hydraulic control systems. When the primary source of seal oil does not produce sufficient seal oil pressure, the backup source provides this pressurized oil. This occurs when a pressure regulator opens to permit oil to flow from the backup system to the seal plates when the pressure differential between the seal oil and hydrogen gas drops to a preset limit. In the existing seal oil systems, the normal operating differential pressure must be sufficiently high to allow early detection and correction of abnormal operation and yet prevent overlapping operation of the primary and backup sources of seal oil. These systems typically operate with a twelve psi pressure differential and require a minimum pressure differential of four psi for maintaining the gas seal. This range of pressure differentials allows sufficient separation between normal and abnormal operation to prevent premature actuation of the backup source of seal oil. Typical of this type of system is the British Pat. No. 1,167,192, which illustrates a standby lubrication system utilizing a hydraulic accumulator maintained at constant pressure for supplying oil in case of a primary lubricating pump failure and a pressure actuated backup pump. This system operates satisfactorily when the allowable range of pressure differentials is larger than two psi. When a smaller allowable range of pressure differentials occurs, utilizing pressure sensing devices for backup pump actuation can result in small system pressure surges causing simultaneous operation of the primary pump and backup pump.
More recently, with the application of hydrogen cooled generators to gas turbine drives, the seal oil system has been combined with the lubrication oil system of the turbine. The lubrication oil pump provides oil to the seal plates through a seal oil pressure regulator. Whatever backup provisions are used on the lubrication system also become the backup sources for the seal plates' oil supply. This type of system results in some simplification in that a separate source of seal oil pressure is not necessary but a relatively high pressure lubrication system is required. These systems typically operate with a pressure differential of 6 psi which is necessary to minimize hydrogen contamination by air entrained in the seal oil which flows along the shaft toward the hydrogen.
When the 6 psi pressure differential is required from entrainment considerations, when there is no independent source of sufficient pressure for seal oil backup, and when the turbine lubrication oil system does not operate at a sufficient pressure to maintain a 6 psi pressure differential at the seal plates, a new seal oil system design, different from past seal oil systems, is required. A major difficulty in the design of a seal oil system with a 6 psi pressure differential is that the allowable pressure range of 6 to 4 psi is nearly impossible to maintain by using pressure sensors only without experiencing either overlapping operation of the primary and backup oil supply systems or seal oil flow interruption to the seal plates resulting in hydrogen leakage and shaft damage.